Can somebody me with some math questions?

in advance, i will give to best answer.

Me! #1 write an equation for the interior angles of this triangle that uses the triangle sum theorem. #2 what is the value of x? #3 what is the measure of #4 classify the triangle above as acute, obtuse, or right. state your reason in a complete sentence.

If f(x)=3x+1 and f^-1=x-1/3, then f^-1(7)=

[10 points, algebra 2]according to my answer key the restrictions are -1 and 4. but why not positive 1?